Thank you for your helpful and prompt reply. Yes, Windows Task Manager is a good idea for monitoring the calculation. I have run it again and it started well, with the CPU usage increasing from 0% to about 10% over 1 minute while the Memory used grew steadily to about 18 GB (of 64 installed) over about 4 minutes. Over this time in Mathematica the Evaluation -> Abort Evaluation option was in black type. Then – quick as a flash – the CPU fell to 0 and the memory to only 132 MB. The Abort Evaluation option was then grey.

So it had died on me! I know 25 days is too long for any computer to countenance, but it should not give up after 4 minutes. The PC has been working OK otherwise.

For me there are now two questions : 1) why is my calculation suddenly ceasing, and without warning or explanation, and 2) would it be better if Mathematica itself provided some run-time information to the user on progress, rather than having to look into Windows Task Manager? Please advise.

Thanks again for suggestions. I have got round the problem by expanding the 11x11 matrix into its 11 10x10 cofactors and evaluating each of these individually, then combining their results. For my purposes I was eliminating selected terms on the way, so finished up with a maxtrix of manageable size. However, I could not do a 12x12 matrix by the same method.

The issue remains as to why Mathematica does not give the user some monitor of progress in large caculations. In my case it ran for a while, used a lot or memory, then suddently gave up and stopped with neither warning nor explanation. In contrast I have another maths algebra package which displays the CPU time used throughout any calculation.